The use of combinations of organic compounds containing at least two intramolecular carboxylic anhydride groups per molecule, particularly copolymers containing succinic anhydride, and polyhydroxyl compounds as binders for paints and coating compositions is already known from EP-A-48 128.
However, the notion of applying the principle of this publication to the reaction of amines with anhydrides is complicated by the fact that the reaction of amines with anhydrides is a very fast reaction, even at room temperature, which leads to crosslinked products with cleavage of the anhydride. The resulting, extremely short pot lives have hitherto prevented polyanhydrides and polyamines from being used together in coating systems.
One possible solution to the problem is disclosed in DE-OS 2 853 477 which describes mixtures of blocked polyamines and polyanhydrides which have good pot lives, but harden on addition of water to form crosslinked products. Ketimines and aldimines obtained by reaction of polyamines with ketones and aldehydes are described as suitable blocked polyamines.
Compounds containing at least two cyclic carboxylic anhydride groups in the molecule, more especially reaction products of polyols with cyclic dianhydrides in a ratio of hydroxyl to anhydride groups of 1:2, or copolymers of an unsaturated cyclic anhydride with polyunsaturated compounds or .alpha.-olefins are mentioned as polyanhydrides.
The olefinically unsaturated compounds to be used in the production of the copolymers are discussed in only very broad terms in the general description of DE-OS 2 853 477; in particular, there is no reference whatever to the quantitative ratios between the individual monomers to be used in the production of the copolymers. The concrete disclosure of the Examples is confined to copolymers of butadiene oil and maleic anhydride in a ratio of 1:1 and to copolymers of wood oil with maleic anhydride. However, these copolymers are attended by disadvantages because their combination with bis-ketimines or bis-aldimines ultimately leads to heavily discolored products. In addition, coating compositions containing unsaturated oils, such as butadiene oil or wood oil, as binder component lead to coatings which tend readily to turn brittle and are not weatherproof
In addition, the Examples of DE-OS 2 853 477 show that dimethylformamide is used as solvent in the processing of the binders specifically described therein, in addition to which unacceptable solids contents of only about 20% are used.
Another possibility of lengthening the pot lives is to use oxazolanes instead of the amines.
Water-hardenable compositions of oxazolanes and polyanhydrides are known in principle from DE-OS 2 610 406 where oxazolanes are combined with polyanhydrides to form water-hardenable sealing compounds and adhesives. Reaction products of polyunsaturated fatty acids with maleic anhydride and polyanhydrides of C.sub.3 -C.sub.6 alkyl (meth)acrylate and maleic anhydride, more especially of butyl acrylate and maleic anhydride, are described as suitable polyanhydrides.
The systems specifically described in DE-OS 2 610 406 are still greatly in need of improvement in regard to their suitability for the production of high-quality, colorless paint films combining hardness with high resistance to solvents and chemicals. This applies both to the systems based on copolymers of maleic anhydride and butyl acrylate which are described in the Examples and to systems based on reaction products of maleic anhydride with polyunsaturated fatty acid esters which lead to yellowing end products.
The use of compounds containing unsaturated (meth)acrylate groups in combination with polyamines for lacquers and coating systems is known, for example, from EP-A 34720, 40288, 165 580, 167 042 and 203 296.
However, the reactivity of binder combinations such as these is in need of improvement. Even where selected catalyst systems are added, the degree of crosslinking is still not sufficient and requires at least very long reaction times, particularly where hardening is carried out at room temperature.
The object of the present invention is to provide new binder combinations which are suitable for the production of high-quality systems of adequate pot life optionally hardening under the effect of atmospheric moisture. The coatings obtained are intended to be clear, colorless, nonyellowing and solvent-resistant.
This object is achieved by the provision of the modified copolymers described in detail hereinafter and of the binder combinations containing the modified copolymers and blocked polyamines of the type described in detail hereinafter.